Modern switching systems may comprise a number of subscriber access units, which are also referred to as remote switching units (RSU), which are controlled by a central control unit (“coordination processor”, CP). The switching units have their own switching network and are also connected via internal lines, that is to say they can also pass on calls between their connections, without having to use lines which pass via the central control unit, although this process is controlled by the central control unit.
The remote switching units of the switching center can be connected via dedicated groups of lines to the surrounding network, that is to say to other switching centers. All the groups of lines from switching units of a switching center which lead to another switching center form a logical group of lines, including the group of lines of the line trunk group of the central control unit, and this connects the switching center to another switching center.
Where an incoming or outgoing call which is intended for a specific other switching center occurs at a switching unit, a switching center such as this may, for example, be a “remote” switching center in another local network or else a switching center in the same local network, the normal procedure is to use a program for line selection to determine all the free lines in the group of lines to the desired switching center and then, for example, to use that line which has not been used for the longest time for that call.
However, this procedure has the disadvantage that, in consequence, calls are passed internally via the switching network and/or internal line via the subscriber access units and/or the central line trunk group, so that this results in an unnecessary internal load on the switching center which, among other things, can lead to delays in setting up connections. Furthermore, this reduces the possibility of there being a free line for a call between two subscriber access units in that switching center.